1. Field of the Invention
The present invention relates to a power amplifier. More particularly, the present invention relates to a power amplifier with an active bias circuit for improving power-added efficiency (PAE).
2. Description of Related Art
A power amplifier (PA) is an important device in microwave circuits. For example, a power amplifier is used to amplify the broadcast signals at the transmission (TX) terminal of a radio frequency circuit. With the popularity of portable communication systems, a maximum output power and output power efficiency have become critical factors for the development of the wireless transmission.
The output efficiency of the power amplifier is represented by the output power and the output power efficiency. Typically, the maximum output power is the most important factor in output power while a power gain and power-added efficiency (PAE) are the two most important factors in output power efficiency. The power gain of the amplifier is defined as a ratio of the output power to an input power. In other words, a larger power gain implies a higher amplifying capacity for the power amplifier. The power-added efficiency is an ability that the power amplifier converts a direct current (DC) input power into an alternating current (AC) output power. In fact, power-added efficiency is defined as the ratio of the direct current (DC) input power to the alternating current (AC) output power, or PAE=(AC output power−DC input power)/DC input power. Similarly, the higher the power-added efficiency, the better the power amplifier is. FIG. 1 is a graph showing the relationship of the output power, the power gain and the power-added efficiency versus the input power of a conventional power amplifier. As shown in FIG. 1, most power amplifiers have excellent output power linearity when the input power is small. However, when the input power is large, the output power is no longer linearly amplified and results in a drop in the gain. This drop in the gain is known as a gain compression. The quality of the power amplifier can be assessed according to the input power at which the gain compression is generated because a later generation of the gain compression means that the power amplifier has better output linearity and maintains a distortion free signal output for a larger input power range.
FIG. 2 is a circuit diagram of a conventional semiconductor transistor power amplifier. As shown in FIG. 2, the transistor 202 serves as a power amplifier transistor and may be such as a CMOS transistor. In FIG. 2, Vg is the gate bias voltage for the transistor 202; Pm is the input power; Pout is the output power. In a conventional CMOS power amplifier, a class-A bias design is often adopted. In the class-A bias design, the transistor 202 is biased at a half of the saturation voltage Vsat thereof so that a full wave transmission of signal is still possible at a larger output power level. Hence, the power amplifier can have a higher output power and a higher power gain as well as a distortion free linear operating region. However, a disadvantage is that most A-class power amplifiers are designed to use a single constant gate bias voltage (for example, the gate bias voltage Vg of the transistor 202 is still at half Vdd). Since output power is equal to a product of voltage and current, power consumption is only reduced by a little at a low output power when the current flow is small and hence leads to an undesirable waste of DC power.
Therefore, in order to increase output the power efficiency of the power amplifier, the power amplifier sometimes operates in a class-AB or class-B operating region. In other words, the gate bias voltage Vg of the transistor 202 in FIG. 2 is lowered. Thus, the power-added efficiency (PAE) of the power amplifier is increased. However, the signal waveform is constrained by the gate bias voltage when the output power is large so that the maximum output power and linearity of the power amplifier are reduced.
Hence, enhancing the power-added efficiency and lower direct current power consumption of the power amplifier without reducing the maximum output power and linearity of operation of the device is important and required.